1. Field of the Invention
This invention relates to a filter for removing an aqueous fluid and entities, such as bacteria, existing in such aqueous fluid, from a nonaqueous fluid.
2. Description of the Related Art
A number of patents are directed toward the combination of a hydrophobic filter and a hydrophilic filter for removing air or other gas from an intravenous fluid before such intravenous fluid reaches a patient. See, e.g., U.S. Pat. Nos. 4,013,072; 4,031,891; 4,116,646; 4,262,668; 4,278,084; 4,515,606; 4,525,182; 4,571,244; 4,615,694; 5,126,054; 5,308,333; 5,439,587; and 5,536,413.
None of the preceding patents, however, applies to a device which filters the fluid first through the hydrophobic filter and then through the hydrophilic filter or vice-versa. In each case, the intravenous liquid comes into contact with a hydrophobic filter through which gas from the liquid may escape and then the intravenous liquid passes through a hydrophilic filter.
Three patents do, though, apply to devices which transmit fluid through both a hydrophobic filter and a hydrophilic filter, viz., U.S. Pat. Nos. 4,026,792; 4,459,139; and 4,938,389.
U.S. Pat. No. 4,026,792 of George Otto Orth, Jr. applies to a xe2x80x9cmethod of treating waste water containing particulate matter, liquid oils and fats to remove the same. . . .xe2x80x9d The disclosure indicates that this waste water is first forced through a hydrophobic filter by centrifugal force. xe2x80x9cFats, oil and oily particulate matter are adsorbed by the hydrophobic filter . . . . The remaining waste water then moves radially into the hydrophilic filter . . . which removes the remaining particulate matter. As indicated in column 3 of U.S. Pat. No. 4,459,139, xe2x80x9cThe key characteristic of the hydrophobic filter membrane is, of course, that it will allow air or other gas to pass therethrough but will block the passage of water or other aqueous liquids. . . . [But] . . . a hydrophobic filter membrane has its own water-breakthrough point, i.e., the amount of pressure differential across the membrane required to drive water through it.xe2x80x9d For the method of U.S. Pat. No. 4,026,792 to function as intended, the centrifugal force must, therefore, be sufficiently large that the pressure differential at least equals the water-breakthrough point.
In U.S. Pat. No. 4,459,139 of Charles E. vonReis and Karlis Vizulis, itself, a device is claimed xe2x80x9chaving a hydrophilic filter in overlying relationship to [a] . . . hydrophobic filter on the inlet chamber side of the hydrophobic filter such that any fluid flow from the inlet chamber to the outlet chamber can only be by passage of the fluid first through the hydrophilic filter and then through the hydrophobic filter. . . . xe2x80x9d Both the hydrophobic filter and the hydrophilic filter have a pore size rating in air of less than 0.5 microns and, preferably, of approximately 0.2 microns; each then xe2x80x9c. . . blocks bacteria from passing. . . . xe2x80x9d These filters, furthermore, preclude liquid from reaching a suction pump used to aspirate liquid from a patient. The fact that a significant pressure differential is created across the combined filters is evident from the following excerpts:
In columns 2 and 3, it is stated that xe2x80x9c[i]n operation the aspirator pulls a vacuum (i.e. a negative pressure) . . . to aspirate fluid from the patient. . . . xe2x80x9d
The vonReis patent, in column 4, further observes, xe2x80x9cThe hydrophobic filter used in the practice of the present invention should preferably have a water-breakthrough point of at least about 10 psi, and ideally above the maximum pressure differential which can be expected, i.e. about 14 psi for the aspirating system described.xe2x80x9d
Columns 1 and 2 of the vonReis patent contain a declaration that, xe2x80x9c[i]t is well known that a hydrophilic filter allows the passage of air therethrough until it is saturated with liquid but blocks or at least substantially restricts the passage of air when it does become saturated with liquid. Where the pressure differential across the hydrophilic filter does not exceed the bubble point of the filter (i.e. the pressure required to force air through the filter when it is saturated with liquid), the passage of air is completely blocked when it becomes saturated. But even where the pressure differential does exceed the bubble point, the hydrophilic filter when saturated will nevertheless substantially restrict the passage of air.xe2x80x9d
And, again in column 4 of the vonReis patent, one reads, xe2x80x9c . . . in a preferred embodiment the hydrophilic filter membrane used had a bubble point of from about 7 to 10 psi and as it reached saturation the blockage of air was about 80%. . . . xe2x80x9d.
U.S. Pat. No. 4,525,182 of Donald B. Rising and Richard G. Naegeli, Jr., in fact, asserts, xe2x80x9cThe typical small pore size of the wetted [hydrophilic] filter prevents gas from passing through said filter at the usual operating pressures.xe2x80x9d Moreover, using almost identical language, U.S. Pat. No. 5,439,587 of Ralph J. Stankowski, Michael C. Heath, and Douglas A. Boucher asserts, xe2x80x9cThe typical small pore size of the hydrophilic filter prevents gas from passing through the filter at the usual operating pressures.xe2x80x9d
The third patent concerning a device which transmits fluid through both a hydrophobic filter and a hydrophilic filter, i.e., U.S. Pat. No. 4,938,389 of Scott R. Rossi and Jeffrey P. Gilbard, claims a reservoir for storing sterile liquids connected to a dispensing tip with a flow passage across which a filter assembly is sealed. The filter assembly comprises xe2x80x9ca hydrophilic filter and a hydrophobic filter arranged in fluid communication serially along said flow passage so that said hydrophilic filter is nearer to said reservoir than said hydrophobic filter, said hydrophobic filter and said hydrophilic filter each having pores sufficiently small to act as a microbial filters.xe2x80x9d
xe2x80x9cIn preferred embodiments of the invention, the filter assembly has the hydrophobic and hydrophilic filters separated, e.g., by a support ring. A more preferred embodiment has a filter structure whereby there are a plurality of support rings between, and on opposite sides of, the filters to provide structural support and filter separation.xe2x80x9d
Since the examples of the Rossi patent utilized an xe2x80x9ceye drop solutionxe2x80x9d as the sterile liquid, since solutions for rinsing a person""s eyes are generally aqueous saline solutions, and since the Rossi patent was not limited to nonaqueous solutions, it is apparent that a significant pressure differential would have to be created across the hydrophobic filter, i.e., the water-breakthrough point would have to be reached, in order to permit the solution to pass through the hydrophobic filter.
It should be noted that none of the preceding patents were intended to remove water from another liquid.
Additionally, lines 11 through 12 in column 3 of U.S. Pat. No. 5,126,054 clarify that the xe2x80x9c[l]iquiphobic layer 18 is superimposed on liquiphilic layer 16. . . . xe2x80x9d Similarly, U.S. Pat. No. 5,536,413 implies that there is no space between the liquiphobic and the liquiphilic layers of the gas venting element of that patent when it states that xe2x80x9c . . . the layers of the gas venting element may be individually prepared and bonded together by various means known to those skilled in the art.xe2x80x9d
Moreover, none of the filter material in the preceding patents combines hydrophilic and hydrophobic characteristics on the molecular level. U.S. Pat. No. 4,031,891 of Thurman S. Jess does state, xe2x80x9cWhile the invention has been described above as using three different filter elements, namely a hydrophilic filter element to cover the central window opening . . . and separate hydrophobic filter elements covering the opposing window openings . . . , it will be understood by those skilled in the art that use can also be made of a continuous sleeve of filter material, the ends of which have been rendered hydrophobic in nature and the central portion of which has been rendered hydrophilic in nature.xe2x80x9d It is, however, apparent that the hydrophilic and the hydrophobic segments of the filter in the Jess patent are distinct from one another on a macroscopic level. This is, also, true for the filter material to which reference is made in U.S. Pat. No. 4,278,084 of J. Lee Pope, Jr.: xe2x80x9c . . . it has been suggested in U.S. Pat. No. 3,520,416 to Keedwell to use a microporous filter material which is hydrophilic in some areas, and hydrophobic, as by the application of silicone treatment, in other areas.xe2x80x9d
The present invention consists of a hydrophilic filter and a hydrophobic filter arranged in fluid communication serially along the flow path of a fluid.
In a first embodiment the pores of both the hydrophilic filter and of the hydrophobic filter are selected to be of such a size that bacteria can not pass through either filter but that a gas such as air can substantially freely traverse the filters.
In a second embodiment the pores can be larger since it is merely desired to prevent an aqueous fluid, such as water, in a nonaqueous fluid, such as gasoline, from passing through the filters with the nonaqueous fluid.
In both embodiments, the hydrophilic filter and the hydrophobic filter could touch one another, but it is preferred to maintain a space between them to accommodate any of the fluid which is desired to be removed that manages to pass the first filter in the flow path but not the second filter. It is, furthermore, preferred to place within such space a structure that will maintain the space between the hydrophilic filter and the hydrophobic filter without significantly restricting the flow of fluid. This facilitates drying of any fluid between the hydrophobic filter and the hydrophilic filter.
In fact, preferably an encasement having an inlet and an outlet contains the hydrophilic and the hydrophobic filters and possesses a spacer to maintain the hydrophobic filter physically separate from the hydrophilic filter. The hydrophilic filter is preferably placed so that in use it will be upstream from the hydrophobic filter. As the encasement, filters, and spacer proceed in the direction that is intended to be downstream, the dimensions of the filters and spacer perpendicular to the intended direction of fluid flow decrease so that as the stream of fluid expands perpendicularly to its intended direction of flow, the possibility of the fluid contacting other than a filter and thereby precipitating some of any aqueous liquid that the fluid may contain is increased.
And an additional embodiment is composed of filter material which has both hydrophilic and hydrophobic characteristics on the molecular level.